1. Field of the Invention
The present invention relates to a dielectric composition and a ceramic capacitor using the same, more particularly relates to a dielectric composition superior in resistance to reduction at the time of sintering, exhibiting a high dielectric constant and low distortion factor, and superior in the temperature-dependence of the capacity and to a ceramic capacitor having a high insulation resistance, high capacity, and low distortion characteristic, having a flat temperature-dependence of the capacity, and using a base metal for the internal electrodes.
2. Description of the Related Art
As a dielectric composition used for a ceramic capacitor, one having a high dielectric constant comprised mainly of barium titanate (BaTiO3) is known, but since it is a strong dielectric, the nonlinear characteristic of the voltage is strong and the distortion factor originate from D-E hysterisys loop is a large xe2x88x9250 dB to xe2x88x9270 dB. Therefore, a BaTiO3 based capacitor cannot be used for a capacitor for which a low distortion factor is required such as a coupling circuit capacity, audio circuit capacitor, or image processing circuit capacitor. Film capacitors, electrolytic capacitors, etc. are exclusively being used for these, but such film capacitors or electrolytic capacitors are hard to reduce in size and there are problems in surface mounting.
On the other hand, among the ceramic capacitors, ones comprised of ordinary dielectrics such as CaTiO3, SrTiO3, CaSrZrO3, and NdTiO3 are low in distortion factor and therefore can be used for coupling circuits, audio circuits, etc., but since they are ordinary dielectrics, the dielectric constant er is a low 30 to 200 and obtaining a high capacity capacitor would be difficult.
Therefore, as a dielectric composition exhibiting both a high dielectric constant and low distortion factor, ones comprised mainly of SrTiO3, Bi2TiO3, CaTiO3, and PbTiO3 have been proposed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 3-97669).
Internal electrodes of a ceramic capacitor, however, use a precious metal such as platinum Pt, gold Au, Pd or silver Ag, and their alloy for example, Ptxe2x80x94Au, Agxe2x80x94Pd but from the viewpoint of cost, nickel or another base metal is preferably used.
The above-mentioned dielectric composition contains bismuth Bi or lead Pb which have a low vapor pressure, so when sintered in a reducing atmosphere, these end up evaporating. Accordingly, while predicated on sintering in an oxidizing atmosphere, if sintered in an oxidizing atmosphere, when using a low cost base metal, for example, nickel, for the internal electrodes, the nickel ends up being oxidized. In the end, there is no choice but to use a precious metal such as platinum, gold, or silver for the internal electrodes.
An object of the present invention is to provide a dielectric composition superior in resistance to reduction at the time of sintering, exhibiting a high dielectric constant and low distortion factor, and superior in temperature-dependence of the capacity and a capacitor having a high insulation resistance, high capacity, and low distortion characteristic, having a flat temperature-dependence of the capacity, and using a base metal for the internal electrodes.
(1) The present inventors et al. engaged in in-depth research in order to obtain a balanced dielectric composition having a large dielectric constant, a small distortion factor, and a small rate of change of the capacity with respect to temperature (hereinafter referred to as a flat temperature-dependence of the capacity) and as a result obtained the following discoveries.
First, the distortion factor depends on the field dependency and non-linearity of the dielectric constant (that is, a strong dielectric property). As measures for suppressing the factor, it may be considered effective to improve the linearity of the dielectric constant or reduce the crystalline anistrophy so as to reduce the strong dielectric property or to use an ordinary dielectric phase. If the strong dielectric property is reduced, however, the dielectric constant falls, so balancing these becomes important. Further, an ordinary dielectric exhibits a negative temperature-dependence of the capacity, while a strong dielectric exhibits a positive temperature-dependence of the capacity, so balancing the strong dielectric and the ordinary dielectric becomes important for the temperature-dependence of the capacity as well.
The balance of the dielectric constant, distortion factor, and temperature-dependence of the capacity becomes suitable by controlling the molar ratios of composition of the calcium titanate, strontium titanate, and barium titanate. In the present invention, by adding the strong dielectric barium titanate to the ordinary dielectric calcium titanate (or strontium titanate), the balance of the ordinary dielectric phase and strong dielectric phase is promoted and it is possible to obtain a dielectric composition having a large dielectric constant, a small distortion factor, and a flat temperature-dependence of the capacity.
That is, if the molar ratio of composition of barium titanate is increased, while the dielectric constant becomes larger, the distortion factor and the rate of change of the capacity with respect to the temperature also become larger. As opposed to this, if the molar ratio of composition of strontium titanate is increased, while the distortion factor becomes smaller, the dielectric constant also becomes smaller. Further, when the molar ratio of composition of calcium titanate is increased too, while the distortion factor becomes small, the dielectric constant also becomes small.
Further, comparing calcium titanate and strontium titanate, if the molar ratio of composition of calcium titanate is increased, the rate of change of capacity with respect to temperature becomes smaller, but the dielectric constant also becomes smaller. As opposed to this, if the molar ratio of composition of strontium titanate is increased, the dielectric constant becomes larger, but the rate of change of the capacity with respect to temperature also becomes larger.
(2) Based on this discovery, the first aspect of the invention provides a dielectric composition containing at least calcium titanate, strontium titanate, and barium titanate, wherein the molar ratios of composition of the three are such that at least a molar ratio of composition of the barium titanate is not more than 0.3 and wherein the composition comprised of the calcium titanate, strontium titanate, and barium titanate contains at least one crystal structure from among tetragonal and orthorhombic crystal structures.
The composition comprising calcium titanate, strontium titanate, and barium titanate exhibits an insoluble phase and cubic crystal structure as well due to these three ratios of composition, but ratios of composition exhibiting mainly an insoluble phase are not desirable since the distortion factor is large, while ratios of composition exhibiting mainly a cubic crystal structure is not desirable since the distortion factor is large and the rate of change of the capacity with respect to the temperature is also large.
Further, a cubic crystal structure is exhibited in the region where the barium titanate is rich, but in this region, the molar ratio of composition of the barium titanate is large, so the dielectric constant becomes large, but the distortion factor also becomes large and the balance between the dielectric constant and the distortion factor is poor. Therefore, a tetragonal crystal structure in the region where the molar ratio of composition of barium titanate is not more than 0.3 is more preferable.
In the above aspect of the invention, the crystal structure of the dielectric composition obtained is not one which is completely comprised of at least one of a tetragonal crystal and orthorhombic crystal structure. It means the main crystal structure is at least one of a tetragonal crystal and orthorhombic crystal structure. Therefore, inclusion of an insoluble phase or cubic crystal structure etc. in part of it is also included in the technical scope of the present invention.
In particular, when the calcium titanate, strontium titanate, and barium titanate are in ratios of composition of an area near the transition point between a tetragonal crystal structure and orthorhombic crystal structure, the balance between the dielectric constant and distortion factor of course and also the temperature-dependence of the capacity (temperature coefficient) become good.
(3) Further, a second aspect of the present invention provides a dielectric composition containing at least calcium titanate and barium titanate, wherein when the calcium titanate is CT and the barium titanate is BT,
(CT)x(BT)y(F)1xe2x88x92xxe2x88x92y(where F is any component)
0.4xe2x89xa6x less than 1, 0 less than yxe2x89xa60.2
are satisfied (see FIG. 2). Note that the third component F is not particularly limited.
By controlling the ratios of composition of the ordinary dielectric calcium titanate and the strong dielectric barium titanate, it is possible to promote the balance of the ordinary dielectric phase and strong dielectric phase and obtain a dielectric composition having a large dielectric constant, a small distortion factor, and a flat temperature-dependence of the capacity.
If the molar ratio of composition (y) of the barium titanate is large, the dielectric constant becomes large, but the distortion factor also becomes large, so to obtain a balance between the dielectric constant and the distortion factor, it is more preferable that 0 less than yxe2x89xa60.2. Further, if the molar ratio of composition (x) of the calcium titanate is small, the dielectric constant becomes large, but the rate of change of the capacity with respect to the temperature also becomes large (temperature-dependence of the capacity deteriorates), so to obtain a balance between the dielectric constant and the temperature-dependence of the capacity, it is more preferable that 0.4xe2x89xa6x less than 1.
(4) A third aspect of the present invention provides a dielectric composition containing at least calcium titanate, strontium titanate, and barium titanate, where at least the three molar ratios of composition are such that
the molar ratio of composition (P) of calcium titanate is 0.5 to 0.85, the molar ratio of composition (Q) of strontium titanate is 0.05 to 0.4, and the molar ratio of composition (R) of barium titanate is 0.1 to 0.2 (however, P+Q+R=1) (see FIG. 3).
By adding the strong dielectric barium titanate to the ordinary dielectrics calcium titanate and strontium titanate, the balance of the ordinary dielectric phase and strong dielectric phase is promoted and it is possible to obtain a dielectric composition having a large dielectric constant, a small distortion factor, and a flat temperature-dependence of the capacity.
If the molar ratio of composition (P) of the calcium titanate is small, the dielectric constant becomes large, but the rate of change of capacity with respect to temperature also tends to become large (temperature-dependence deteriorates). Further, if the molar ratio of composition (Q) of the strontium titanate is large, both the distortion factor and the rate of change of capacity with respect to temperature tend to become large. Further, if the molar ratio of composition (R) of the barium titanate is large, the dielectric constant becomes large, but the distortion factor also tends to become large. Therefore, to obtain a balance of the dielectric constant, distortion factor, and temperature-dependence of the capacity, it is preferable that 0.55xe2x89xa6Pxe2x89xa60.8, 0.2xe2x89xa6Qxe2x89xa60.35, and 0.12xe2x89xa6R xe2x89xa60.18, in particular that the region be near the transition point between the tetragonal crystal and orthorhombic crystal structures.
(5) Note that in the second and third aspects of the invention, it is preferable that the dielectric composition include at least one crystal structure from a tetragonal crystal and orthorhombic crystal structure. Ratios of composition exhibiting mainly an insoluble phase are not preferable since the distortion factor becomes large, while ratios of composition exhibiting mainly a cubic crystal structure is not preferable since the rate of change of capacity with respect to temperature also becomes large.
(6) The first to third aspects of the invention explained above may also contain various additives.
(6-1) As the type of additives, MnO, CrO, and other reduction resistance aids may be mentioned. These reduction resistance aids have the effect of promoting sintering and the effect of improving the insulation resistance (IR) but if added in large amounts, the insulation resistance, dielectric loss (tan xcex4), and distortion factor deteriorate, so the amount is preferably made 0.1 to 1 mol %.
(6-2) Further, as other additives, a glass composition or other sintering aid may be mentioned.
As a sintering aid preferably used in the present invention, at least one type of glass composition selected from the G group of SiO2 and Al2O3, M group of BaO, CaO, and SrO, and L group of Li2O, Na2O, K2O, and B2O3 may be mentioned.
In the dielectric composition of the present invention, if the sintering temperature is not more than 1340xc2x0 C., a sufficient sintered body cannot be obtained. If sintered at under this temperature, a reduction in the dielectric constant and a reduction in the insulation resistance IR occur and the distortion factor increases. On the other hand, the additives SiO2, Al2O3, V2O5, MoO3, WO3, and Co3O4 have the side effect of causing a reduction in the sintering temperature, but if the amounts added are increased, have the defect of causing a reduction in the insulation resistance IR. In general, lowering the sintering temperature has the effect of improving the cover rate of the internal electrodes, the effect of suppressing their becoming too thick, and other defects and the effect of suppressing the oxidation and dispersion of the internal electrodes. Therefore, by adding a sintering aid as in the present invention, the state of the internal electrodes becomes good and the reliability of the break down voltage etc. are improved. Further, there is not that much of an effect on the temperature-dependence of the capacity by the addition of the sintering aid.
When added in a large amount, however, unevenness occurs in the sintering ability and the insulation resistance and distortion factor deteriorate, while when too little is added, the effect of low temperature sintering is lost, so a range of 0.2 to 5 mol % is preferable.
When using at least one type of glass composition selected from the above G group, M group, and L group as the sintering aid, preferably, when the ratios of composition of the G group, M group, and L group are expressed by a triangular diagram (G, M, and L), the ratio of composition of the sintering aid is in the area surrounded by the following points X1 to X5 (including on lines):
X1: (0.0, 0.0, 1.0)
X2: (0.0, 0.5, 0.5)
X3: (0.1, 0.65, 0.25)
X4: (0.5, 0.0, 0.5)
X5: (0.65, 0.05, 0.3)
Further, more preferably, when the ratios of composition of the G group, M group, and L group are expressed by a triangular diagram, the ratio of composition of the sintering aid is in the area surrounded by the following points X1, X6, X7, and X5 (including on lines):
X1: (0.0, 0.0, 1.0)
X6: (0.0, 0.2, 0.8)
X7: (0.3, 0.4, 0.3)
X5: (0.5, 0.0, 0.5)
(6-3) As other additives, it is possible to mention at least one oxide selected from V2O5, MoO3, WO3, and Co3O4. These have the effect of lowering the sintering temperature and lowering the distortion factor. Among these, V2O5 is most preferable. If these are added in large amounts, the insulation resistance and the dielectric loss remarkably deteriorate, so it is preferable to add them in the range of 0.01 to 0.5 mol %.
(6-4) Further, as other additives, it is possible to mention at least one oxide selected from Nb2O5, Ta2O5, Y2O3, La2O3, CeO2, Gd2O3, Dy2O3, and Ho2O3 These have the effect of improving the dielectric loss and reducing the distortion factor. These are preferably added in the range of 0.01 to 0.2 mol %.
(7) The dielectric composition according to the above present invention is preferably used as a material of the dielectric layer of a ceramic capacitor having internal electrodes and the dielectric layer.
In this case, the internal electrodes are more preferably comprised by Ni or an Ni alloy. A dielectric composition according to the present invention is superior in reduction resistance, so can be sintered in a reducing atmosphere, can use Ni or an Ni alloy for the internal electrodes, and can be reduced in cost.
Note that the structure of the ceramic capacitor etc. is not particularly limited and includes not only a stacked type capacitor, but also a single sheet type capacitor.